Gaseous conduction device



1941- c. G. SMITH 2,253,145

GASEOUS CONDUCTION DEVICE Original Filed May 24, 1926 CAES/UM CHLOE IDE 17706772??? CH/IEL-ES 6. SMITH Uormfg Patented Aug. 19, 1941 GASEOUS CONDUCTION DEVICE Charles G. Smith, Medford, Mass., assignor to Raytheon Manufacturing Company, Newton, Mass a corporation of Delaware Original application May 24, 1926, Serial No. 111,278. Divided and this application March 11, 1937, Serial No. 130,268

2 Claims,

This invention relates to devices of .the type having a tube containing electrodes separated by a gaseous medium, preferably at a low pressure, and particularly to gaseous conduction rectifiers, objects of the invention being to produce a device of this type having low total voltage loss, little if any cathode disintegration, small reverse current when used as a rectifier, little if any gaseous clean-up, protection against deleterious effects of ions and radiation upon the insulating material embodied in the tube, low starting voltage, stability of operation, and generally to improve devices of the aforesaid type.

This is a division of my Patent No. 2,077,961, dated April 20, 1937, for an improvement in Gaseous conduction devices.

In one aspect the invention comprises a gaseous conduction tube having an anode, a cathode, and a cathode coating of caesium or other alkali metal or alkali earth metal or other material characterized by a low work function, a low vaporizing temperature, and/or being highly electropositive, the cathode being constructed and arranged to operate at a temperature below that at which the coating is driven oil and the anode being constructed and arranged to operate at a temperature above said temperature, whereby coating material deposited upon the anode during manufacture or during operation is driven off and the action of the material is confined to the cathode. Thus, when used as a rectifier, for example, the tube permits current to flow freely when the cathode is negative while permitting practically no reverse current when the anode is negative. While some of the material may be driven off the cathode, particularly when operating under conditions which cause the discharge to are from a hot-spot, the cathode is sufficiently large to permit the discharge continuously to shift to coated areas. The cathode coating serves to facilitate the gaseous conduction and to lower the potential drop in the region of the cathode, thereby lowering the total potential drop between cathode and anode.-

By making the cathode large it can be maintained at an average temperature well below that at which the coating is rendered ineffective and by making the cathode hollow the coating material which is driven off may redeposit upon other areas so that even in the case of a wander- I ing arc each spot from which the coating is driven oif is soon recoated by material driven off other spots. However, the construction of the cathode and the gas filling may be correlated to maintain the discharge without a hot-spot on the cathode, in some cases in the form of a flow discharge and in other cases in the form of an are without a hot-spot, as disclosed in Patent No. 1,929,123, dated October 3, 1933. The anode may be heated by an auxiliary source of heat, but the preferred way of keeping the anode free from the coating material is to make it so small that it normally operates at a temperature above that at which the coating will adhere.

In some cases, as when using caesium, the cathode coating may be very thin, even to the extent of being invisible. Indeed, for certain purposes satisfactory results may be obtained with the coating in the form of a layer of the vapor of the material adsorbed on the surface of the cathode, the vapor not only increasing the electron emission when deposited in or on the surface of the cathode but also affording a copius supply of ions in the gap between cathode and anode by virtue of its low ionization voltage. of the vapor materials above referred to, caesium has the advantage of a low vaporization temperature, this metal having an appreciable vapor pressure even at room temperature. 4

The cathode coating adheres better and is more effective if the cathode surface is rough. An

effective surface is formed by making the cathode of carbon or depositing a layer of carbon on a metal cathode before applying the caesium or other coating. Carbon is more electronegative than most surfaces and therefore serves better to bind the caesium or other electropositive material to its surface. The cathode, particularly when formed of metal instead of carbon, may be coated with an oxide, for example, an oxide of an alkali earth metal, or with chromium, before applying the electropositive material, such as caesium. Or, the cathode, if formed of metal, may merely be oxidized to make its surface highly electronegative.

In another aspect, the invention comprises the use of an inert gas with caesium or other of the aforesaid materials which have vapor pressures too low, at normal tube temperatures, alone to maintain gaseous conduction. The inert gas serves for starting purposes, and when ionized by the discharge, its radiation ionizes the vapor of caesium or the like, affording a very low voltage drop. When using caesium, neon is particularly effective for the reason that the radiation from glowing neon is a powerful ionizing agency for caesium vapor. This combination of gas and vapor is particularly useful in gaseous rectifiers since the vapor alone, at temperatures high enough to produce suificient vapor pressure" to maintain conduction, would tend to permit current flow in both directions between cathode and anode.

I have found that vapors, such as caesium, tend to react with the materials of the tube, particularly the glass container and other insulation parts, when ionized by radiation of ionized inert gas or otherwise, and I therefore prefer to shield such parts by confining the discharge. The best way of accomplishing this shielding, so far as I am now aware, is to make the cathode hollow and to present the anode to the interior surface of the cathode so that most, if not all, of the radiation is confined to the interior of the oathode. The anode may extend through a restricted opening in the cathode with means adjacent the opening to obstruct the small amount of radiation which would otherwise emanate through the opening, and the spacing between the anode and cathode opening and/or between one or both electrodes and the obstructing means may be restricted substantially to the mean free path of electrons in the gas for insulation purposes. By thus confining the radiation, it assists in maintaining a higher degree of ionization, thereby lowering the voltage drop between cathode and anode. Indeed by conserving the radiation in sufilcient degree, the discharge may be caused to assume the characteristics of arc without heating the cathode to the usual high temperature required to maintain an arc by thermionic emission from a hot-spot, as more fully explained in the aforesaid application, Serial No. 111,279, filed May 24, 1926. Still another advantage in making the cathode hollow consists in that the coating material which is vaporized or otherwise disloged from any area of the cathode is redeposited upon other areas of the interior surface of the hollow cathode, thereby conserving the coating material and restricting its escape to other parts of the tube where its effect is deleterious.

Owing to the fact that a highly electropositive material, such as caesium, readily combines with moisture and other constituents of the atmosphere, it should not be introduced into the tubeuntil the tube has been evacuated or until the air has been replaced with inert gas. Consequently, I propose to incorporate, within a confined space including the interior of the tube, such as an integral appendix, ingredients which react to produce the material, and then after the tube is evacuated cause the ingredients to react by application of heat or otherwise, to produce the material in the form of a vapor which deposits upon the surfaces inside the tube. To produce caesium, I preferably employ calcium and caesium chloride. By placing the ingredients in a metallic cup or capsule, they may be heated by current induced in the capsule from a high frequency field generated outside the tube.

For the purpose of illustrating the invention characterized as above outlined, one concrete embodiment is shown in the accompanying drawing, in which,

Fig. 1 is an axial section of a tube;

Fig, 2 is a section on line 22 of Fig. 1;

Fig. 3 is a section on line 33 of Fig. 1;

Fig. 4 is a side elevation of the tube shown in Fig. 1 before it is completed Fig. 5 is a side elevation of a cathode-anode assembly as viewed from the left of Fig. 1;

Fig. 6 is a detail view of the filling capsule; and

Fig. '7 is a detail view of a modification.

The particular embodiment chosen for the puranodes.

pose of illustration comprises a tube l of lime glass or other suitable material, a cathode 2, two anodes 3, leads 4 to the anodes respectively, a lead I to the cathode, a shield 6, a support I for the' shield, and a support 8 for the cathode.

The cathode comprises a cylindrical wall 8 and ends [0 and II. One of the ends may be formed integrally with the cylindrical wall, but they are both preferably formed separately and mounted over the ends of the wall. The ends may have cylindrical flanges which fit over the cylindrical wall as shown at l0 and H' in Fig. 5, or they may be in the form of disks as shown in Fig. 1. In either case they are preferably welded to the cylindrical wall, either continuously around the periphery or at circumferentially spaced spots. The lower end II has two openings with exterior flanges surrounding the reduced ends of the anodes in spaced relation thereto, the outer diameter of the flanges preferably being approximately equal to the diameter of the larger portions of the Mounted within the cathode are rings or washers 12 having integral flanges to hold them in spaced relation, the spaces between the washers being very small to provide crevices from which the electronic discharge emanates more freely than from a flat surface. In the illustrative embodiment, the cathode is approximately one inch in diameter, thus indicating the order of spacing between the washers l2. The washers may be mounted in the cathode in any suitable manner, as for example, merely by a tight fit within the cylindrical portion of the cathode.

As explained above, the interior surface of the cathode is preferably rough, and is preferably coated with caesium or other material having a low work function. The rough surface may be provided merely by oxidizing the surface of the cathode or by first coating it with carbon or with a layer of oxide, as elsewhere explained. Such a surface will adsorb more of the caesium or other electropositive vapor than will a smooth surface. The cathode may be formed of nickel, iron, or more refractory metal, such as tungsten or molybdenum, or it may be formed of carbon, in which case it naturally has a rough surface without being coated on the inside, although even in that case it may have a coating of oxide.

The anodes may be formed of nickel, iron, or more refractory metal, and, as illustrated in Fig. 1, the lead-in wires .4 extend through axial openings in the ends and are secured to the anodes at the upper ends by being pinched in the tips of the anodes. As shown in Fig. 1, the anodes rest upon hollow protuberances upon the reentrant stem 13 of the tube l.

The shield 6 is preferably formed from a single piece of sheet metal, as shown in Fig. 3. The space between the shield and each anode, as well as between the shield and each flangeof the cathode, is preferably confined approximately to the mean free path of electrons in the gas, for insulation purposes. The space between each cathode flange and each anode is also preferably of the same order for the same purpose. The shield 6 not only serves for insulation purposes, but also obstructs radiation from the interior of the cathode to any part of the tube l, including the junction between the tube and the anode where disintegration would otherwise occur by interaction between the caesium and the tube material.

The air within the tube is preferably replaced with an inert gas, such as neon. For low voltage tubes the neon pressure may be' of the order of mm. while for higher voltage tubes the pressure should be somewhat lower. The deposit of caesium or the like, above referred to, is indicated in Fig. 1 on the inside of the tube and on the inside of the cathode by stippling.

One methodof filling the tube is illustrated in Fig. 4 wherein l4 indicates a pump connection. 15 connection to a source of inert gas, and iii an integral appendix on the tube l. Mounted within the appendix I6 is a metallic capsule ll containing ingredients which react to produce the metallic vapor, such as caesium. As shown in Fig. 6, this capsule may contain calcium and caesium chloride, the calcium being somewhat in excess of the amount necessary'to react with all of the caesium chloride to produce caesium.

In order to hold the two ingredients in separated juxtaposition, thereby to prevent their interaction except at very high temperature, they are preferably separated by a partition which permits the ingredients to intermingle only at high temperature. As shown in Fig. 6, the partition is formed as a wad of steel wool through which the ingredients may pass when melted or vaporized. A similar wad is preferably placed at the mouth of the capsule normally to hold the materials in place.

During the pumping operation, the inert gas is I shut off and the tube is heated to drive off impurities.

Current may be passed between the cathode and anodes further to assist in driving off the impurities. After the tube is thoroughly evacuated, the inert gas and caesium are admitted. While the inert gas may be'admitted first, it is preferably first to distill the caesium into the tube inasmuch as it passes in more free 1y at the lower pressure within the tube. After both the inert gas and the caesium or the like have been admitted into the tube, the connections are sealed oif at l8 and I9, as indicated in Fig. 1.

While the ingredients in the capsule I! may be caused to react in various ways, the best way of which I am aware is to heat the ingredients by current induced in the metallic capsule I! from a high frequency source, such as indicated at 20, outside the tube. chloride melts at approximately 647 C. and calcium at about 810 0., the latter subliming readily at 725 C., the whole tube or at least the capsule I1 is preferably heated only to approximately 600 C. to drive off impurities during the pumping operation, and subsequently to approximately at 850 C. to cause the ingredients to react to give off caesium vapor. When the capsule I! is mounted in an appendix, as shown in Fig. 4, most of the caesium vapor condenses in the bottom of the tube, as indicated at 2|, only a small part passing up into the body of the tube, in which case the deposit at 2| may be redistilled throughout the tube by baking the lower part of the tube.

Inasmuch as caesium mounted in the body of the tube, as for example, on the cathode 9, as illustrated at II in Fig. 5, in which case the caesium vapor penetrates to the interior of the cathode through the restricted openings in the lower end of the cathode more readily. In this case the capsule is not removed.

As shown in Fig. 7, the capsule i1 is mounted within the cathode, and may be heated either by current induced in the cathode by high frequency current around the tube or by passing a discharge between cathode and anodes. This method has the advantage of depositing most of the caesium or the like upon the interior of the cathode; and by heating the'cathode hot enough, the coating material may be caused to combine chemically with the cathode, particularly if the latter is formed of nickel. The cathode shown in Fig. 7, as well as that shown in Fig. 5, may have interior surfaces corresponding to washers l2 in Fig. 1. Instead of using calcium, magnesium may be used; and instead of caesium, potassium or the like may be used.

The particular embodiment of the invention shown in the drawing and described in detail is included merely to illustrate the invention, my intention being not to limit myself to this embodiment but only to the generic invention defined by the appended claims.

What is claimed is:

1. A unidirectional gaseous discharge tube comprising a hermetically sealed vessel containing a gaseous atmosphere arranged to be ionized to maintain a glow discharge, a cathode constituting a hollow chamber with an interior electrode surface enclosing a portion of said atmosphere in said vessel, the interior of said cathode chamber having an increased electrode surface formed of a plurality of closely-spaced, projecting wall portions constituting a plurality of narrow, intercommunicating cells terminating into a common space within said hollow chamber, and a small-area anode disposed outside said cells and arranged to maintain a'glow discharge through said atmosphere with the interior conducting surfaces of said cathode through said common space.

2. In a crater type glow discharge tube, a crater cathode comprising a plurality of parallel disks having alined perforations forming the crater,

said disks being spaced to provide a non-re-' 

